As I understand it, Pathos makes two types of output stages. For
class AB working, they use complementary MOSFETs in push-pull.
For single ended (or balanced SE) class A, they use their "INPOL"
circuit, described on their web site at www.pathosacoustics.com.

From descriptions I've read, I gather that INPOL is basically a
source follower loaded by a choke and coupled to the load by a
capacitor. The choke and cap together form an energy storage
system that supplies most of the dynamic output current, while
the source follower MOSFET controls the output voltage.

The key claim for this approach is that it allows the follower to
work under nearly constant-current conditions, so it can control
the output voltage with great accuracy.

Doing some more digging on this, I unearthed a review of the
Pathos Twin Towers amp by Dan Sweeny in Issue 116 (Feb '99)
of The Absolute Sound. In a sidebar on INPOL theory, he explains:

"Under no-signal conditions, the choke allows direct current to
flow unimpeded from the positive supply to ground [while] the
parallel capacitor ... provides additional energy storage while
equalizing the current draw for either half of the wave cycle, i.e.,
when the [output] MOSFET is sourcing or sinking current ...

"Impedances are set so that the total series impedance of the
capacitor plus the loudspeaker circuit is somewhat higher than
the impedance to ground; therefore current through the output
transistor never varies appreciably even though the internal
impedance of the MOSFET drops with increasing signal voltage."

In an effort to see if this could possibly be true, I quickly modeled
the INPOL output stage in Electronic Workbench. In this model at
least, the current through the MOSFET does vary by about 2:1 at
full output, just as it would with a constant current source load.
This appears to negate the claim of constant current operation.

This kind of choke loading does offer the benefit of cutting the
idle dissipation of the output stage in half when compared to a
CCS loading, as there is no need for a corresponding negative
power supply rail.

"Impedances are set so that the total series impedance of the
capacitor plus the loudspeaker circuit is somewhat higher than
the impedance to ground; therefore current through the output
transistor never varies appreciably even though the internal
impedance of the MOSFET drops with increasing signal voltage."

That statement makes no sense to me at all, particularly
in reference to the Pathos schematic.